Answer
Hexaaquachromium(III).
Work Step by Step
Name for the neutral ligand $H_{2}O$ is aqua.
Number of $H_{2}O$ ligands is 6 for which we use hexa.
The complex ion is a cation, so we name the metal same as the element, i.e, chromium.
Oxidation state of the metal is 3 which is indicated by Roman numeral in parenthesis. So the IUPAC name of $[Cr(H_{2}O)_{6}]^{3+}$ is hexaaquachromium(III).
